Process for producing stainless steel and alloy thereof



Dec. 3, 1957 w. MOORE 2,81 ,2

PROCESS FOR PRODUCING STAINLESS STEEL AND ALLOY THEREOF Filed Oct. 2, 1955 3 A+ I200F 8 Sh'ess A+ I300F Rup+ur2 Tim-e in Hours IN VEN TOR. J [Ware ATTORNEY United States Patent PROCESS FOR PRODUCING STAINLESS STEEL AND ALLOY THEREOF James H. Moore, Swampscott, Mass., .assignor to National Research Corporation, Cambridge, Mass, a corporation of Massachusetts Application October 2, 1953, Serial No. 383,753

3 Claims. (Cl. 75-49) This invention relates to the production of metals and in particular to improved high temperature alloys. This application is a continuation-in-part of my co-pending application Serial No. 267,670, filed January 22, 1952, now Patent No. 2,776,204, granted January 1, 1957..

A principal object of the invention is to provide improved low carbon and low nitrogen stainless steels of the 300 series and in particular to stainless steels of the 304 and 316 type.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description.

The present invention is primarily directed to improved stainless steels of the A. I. S. I. designated 300 series and in particular to stainless steels of the 304 and 316 type. The American Iron and Steel Institute has designated as the 300 series those stainless steels which contain bothchromium and nickel, where the total con tent of these elements is at least 23%, with the minimum content of chromium being 16% and the minimum con tent of nickel being 6%. This series includes alloys containing about 16% to 26% chromium and about 6% to 22% nickel. Standard A. I. S. 1. types 304 and 316 stainless steels contain 0.08 and 0.10% maximum carbon respectively. When employing these steels in welding operations, it was found that a harmful carbide precipitation occurred adjacent to the welds so as to reduce the corrosion resistance of those areas. The recent introduction of commercial extra low carbon 300 series stainless steels containing a maximum of 0.03% carbon has brought about some improvement in the Welding characteristics and corrosion resistance of such alloys.

Despite improvements which have been achieved by the extra low carbon 300 series stainless steels, these steels are still inadequate, particularly with respect to rupture life at temperatures above 1000 F. The present invention is based on the discovery that the total carbon and nitrogen content in iron and iron alloys 'has a tremendous effect upon rupture strength at elevated temperatures. For example, two melts of high-purity iron, having low oxygen content but diiferent total carbon and nitrogen content, demonstrate enormously different rupture strength as illustrated in the following table:

TABLE I 50-hour rupture stress The same effect applies to the alloys of the 300 series. Thus, while some improvement in corrosion resistance,

welding characteristics and rupture strength is achieved by using 300 ser'iesalloys of 0.03% carbon, much greater improvements are achieved by maintaining much lower percentages of carbon and much lower percentages of nitrogen. When the total content of both carbon and nitrogen is below about 0.010%, and preferably about 0.006% or less, greatly improved results are obtained. In a preferred embodiment of the present invention, the stainless steels contain less than about 0.006% carbon and less than about 0.005% nitrogen. Stainless steels in the present invention are preferably produced by the refining techniques described in my above-mentioned Patent No. 2,776,204. This technique involves the measurement of the carbon monoxide evolution rate so as to obtain minimum concentrations of carbon, oxygen and nitrogen. Several preferred methods of practicing the invention are set forth in the following .nonlimiting examples:

EXAMPLE I A charge of iron, in the form of 220 lbs. of electrolytic iron stock, is added to a magnesia crucible. To this iron charge there is added approximately 70 grams of carbon in either massive or powder form, but preferably the former. The furnace containing the crucible is then evacuated to a low free air pressure on the order of .001-.010 mm. Hg abs. When this low pressure is obtained, the iron charge in the crucible is heated as rapidly as possible, such as by the use of an induction coil, to a preferred refining temperature on the order of about 1650 C. As a result of this heating, a pressure rise is caused by the very rapid evolution of carbon monoxide. When the temperature has leveled off at about 1650" 0., carbon monoxide evolution rates are determined about every five minutes. This reading may be readily obtained by valving oif the pumping system and measuring the pressure rise. This pressure rise is directly related to the carbon monoxide evolution rate .(E) and can be calculated from the following formula:

where n is the pressure rise in microns Hg abs. per minute, V is the effective volume of the vacuum system in cubic feet, and W is the weight of the iron charge in pounds.

When theevolution rate (E) has fallen to less than about 20 micron cubic feet per minute per pound of iron, preferably less than about 18 micron cubic feet per minute per pound of iron, the vacuum furnace is flooded with an inert gas pressure of between 50 mm. and 760 mm. Hg abs. There are then added to the molten iron the alloying elements necessary to give a final steel composition within the following composition range based on the weight of the iron charge:

Chromium 18.00-20.00%. Nickel 8.004000%. Carbon. .006% maximum. Nitrogen .005 maximum. Oxygen .03% maximum. Iron Balance.

EXAMI'LE II The techniques described in Example I are employed, with the exception that, in this case, the alloying constituents are added to give a final steel composition within the following composition range based on the weight ofthe iron charge:

Chromium 16.0018.00%. Nickel 1000-14.00%. Molybdenum 2.003.00%. Carbon .006% maximum. Nitrogen .005% maximum. Oxygen .03% maximum. Iron Balance.

This gives a steel having the composition specified for type 316 stainless steels.

The steels produced by the above examples have very low total carbon and oxygen contents, being less than 0.01% and, in many cases, having a combined total of 0.006% carbon and nitrogen.

Rupture tests conducted above 1000 F. revealed large differences in the rupture strength between vacuum melted 300 series stainless steels and commercial fextra low carbon 300 series stainless steels. Comparative stresses for specified rupture times are given in the accompanying drawing which shows rupture stress plotted against rupture life at 1000 F., 1200 F. and 1300 F. Curves A A and A on the graph represent 304 stainless steel made in accordance with the present invention and containing a combined total of 0.006% carbon and nitrogen, while curves B B and B represent commercial extra low carbon 304 stainless steel containing a combined total of 0.07% carbon and nitrogen. As can be seen from the graph, at 1000 F. and at rupture time of 100 and 1000 hours, curve A shows stress values of 38, 500, and 31,500 respectively, while curve B shows stress values of 37, 500, and 31,700 respectively. At 1200 F. and at rupture times of 100 and 1000 hours, curve A shows stress values of 24,000 and 19,500 respectively, while curve B shows stress values of 19,500 and 14,500 respectively. At 1300" F. and at rupture times of 100 and 1000 hours, curve A shows stress values of 18,000 and 14,800 respectively, while curve B shows stress values of 13,500 and 9,000 respectively.

Thus, it can be seen that the very low carbon-plusnitrogen alloys of the present invention show a substantially higher rupture life above 1000" F. than the commercial extra low carbon 300 series stainless steels.

The greatly increased rupture life ofthe 304 type alloy of the present invention has been shown to be due to the formation of a stable acicular ferritic structure which results from the high purity of these alloys. The corresponding extra low carbon" 304 alloys which are commercially available have the austenitic structure usually associated with the 300 series stainless steels.

In the specific examples given above, the nickel, chromium and molybdenum alloying elements are added to the melt after the iron has been purified. In this case, it is apparent that the alloying constituents must be of high purity so as not to contaminate the purified iron with carbon, nitrogen and oxygen. In an alternative method of practicing the invention, the nickel and/or molybdenum may be added to the iron charge so as to be purified along with the iron. However, in all cases, it is preferred to add the pure (electrolytic) chromium to the iron after purification of the iron, since this gives a much cleaner melt.

Since certain changes may be made in the above process and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An improved vacuum melted stainless steel alloy containing at least 16% chromium and at least 7% nickel, said steel containing less than about 0.006% total nitrogen and carbon less than about 0.03% oxygen, and having a rupture life of 1000 hours at a temperature of 1200 F. and at a rupture stress of 18,000 pounds per square inch.

2. A vacuum melted iron alloy containing by weight, from about 16% to 18% chromium, 10% to 14% nickel, 2% to 3% molybdenum, less than about 0.006% carbon, less than about 0.005% nitrogen and less than about 0.03% oxygen, the balance being substantially all iron.

3. The process of manufacturing a high purity iron alloy having a content, by weight, of less than about 0.015% oxygen, less than about 0.006% carbon and less than about 0.005% nitrogen, said process comprising the steps of melting iron in a vacuum furnace, said iron being confined by a crucible predominating in an oxide of a metal having a high vapor pressure at the temperature of the molten iron, adding to said iron, at some stage in said process, an amount of carbon in excess of the amount needed for stoichiometrically reacting with the oxygen in said iron, evacuating said furnace, heating said iron and carbon to a temperature on the order of about 1650 C., maintaining said temperature substantially constant, measuring the rate of evolution of carbon monoxide from said molten iron resulting from reduction of iron oxide by said added carbon, arresting the reaction between the molten iron and the crucible at some point between the time when the carbon monoxide evolution rate falls to about 20 micron cubic feet per minute per pound of iron and before the carbon monoxide evolution rate falls to about 4 micron cubic feet per minute per pound of iron, and adding at least one alloying element to said deoxodized iron to form a high purity iron alloy containing 16% to 26% chromium and 6% to 22% nickel.

References Cited in the file of this patent UNITED STATES PATENTS German Oct. 8, 1946 Nisbet Aug. 14, 1951 OTHER REFERENCES 

3. THE PROCESS OF MANUFACTURING A HIGH PURITY IRON ALLOY HAVING A CONTENT, BY WEIGHT, OF LESS THAN ABOUT 0.015% OXYGEN LESS THAN ABOUT 0.006% CARBON AND LESS THAN ABOUT 0.005% NITROGEN, SAID PROCESS COMPRISING THE STEPS OF MELTING IRON IN A VACUUM FURNACE, SAID IRON BEING CONFINED BY A CRUCIBLE PREDOMINATING IN AN OXIDE OF A METAL HAVING A HIGH VAPOR PRESSURE AT THE TEMPERATURE OF THE MOLTEN IRON, ADDING TO SAID IRON AT SOME STAGE IN SAID PROCESS AN AMOUNT OF CARBON IN EXCESS OF THE AMOUNT NEEDED FOR STOICHIOMETRICALLY REACTING WITH THE OXYGEN IN SAID IRON, EVACUATING SAID FURNACE, HEATING SAID IRON AND CARBON TO A TEMPERATURE ON THE ORDER OF ABOUT 1650*C., MAINTAINING SAID TEMPERATURE SUBSTANTIALLY CONSTANT, MEASURING THE RATE OF EVOLUTION OF CARBON MONOXIDE FROM SAID MOLTEN IRON RESULTING FROM REDUCTION OF IRON BY SAID ADDED CARBON, ARRESTING THE REACTION BETWEEN THE MOLTEN IRON AND THE CRUCIBLE AT SOME POINT BETWEEN THE TIME WHEN THE CARBON MONOXIDE EVOLUTION RATE FALLS TO ABOUT 20 MICRON CUBIC FEET PER MINUTE PER POUND OF IRON AND BEFORE THE CARBON MONOXIDE EVOLUTION RATE FALLS TO ABOUT 4 MICRON CUBIC FEET PERR MINUTE PER POUND OF IRON, AND ADDING AT LEAST ONE ALLOYING ELEMENT TO SAID DEOXODIZED IRON TO FORM A HIGH PURITY IRON ALLOY CONTAINING 16% TO 26% CHROMIUM AND 6% TO 22% NICKEL. 